This invention relates to an improvement in prilling whereby water is atomized into a prill tower to effect faster solidification of the prill, and in particular, the outside surface of the prill by providing an increased rate of surface cooling while reducing the required air flows in the tower. This results in a major reduction in air pollution potential in the form of both fume and particulate while at the same time allowing substantial production rate increases. Specifically, the invention relates to quick freezing the outside surface of the prill by flash evaporation of finely atomized water particles on the surface of the prill or in extremely close proximity to the prill to greatly lower the vapor pressure at the surface of the prill and by continued flash evaporation to enhance cooling so as to allow upward flowing air to be reduced to a point where micro-prills which are formed in prilling can settle in the prill tower while obtaining the same or increased production rates. More specifically, the present invention relates to an improvement in melt prilling of water sensitive materials, in particular, ammonium nitrate, urea, potassium nitrate and other water soluble melts which normally absorb water or go into solution readily at ambient conditions in their solid state.
Prilling is a form of granulation where a melt composed of one or several ingredients are formed into small drops and allowed to solidify as they fall by gravity through a gaseous medium which cools them to solidification.
In a prill tower, molten materials are formed into drops by a variety of ways including extruding through holes in concave, convex or flat drilled plates, spray nozzles, and spinning buckets with properly drilled holes. The preferred drilled plates are concave on the reservoir side with a convex surface on the extruded side. They can also be flat with holes drilled at an angle to radiate the streams of prill forming sprays. For purposes of this specification, all methods of prill forming or granulation shall be designated "extruding." As the droplets (prills) form, there are two types of potential air emissions which result.
1. During the formation of prills of the proper size, there are particulate micro-fine prills formed of almost an equal number. These particulate micro-prills are usually emitted out the top of the tower either directly to the atmosphere or into a pollution abatement device.
2. Prill formation creates a lot of surface area of solidifying melt, so the vapor pressure of that melt becomes very important. Vapors are given off from the melt at rates determined by laws of mass transfer. The vapors then form fumes which provide both a particulate and an opacity emission problem. The higher the temperature of the prill surface, and the longer it is at high temperatures, the greater the vapor pressure of the prill, and the more vapor which is given off, forming even more fumes.
Preferably, air flows upward through the prill tower, but there are down flow and cross flow towers also. The air provides an uplift to the prills causing them to settle in the tower at a slower rate than if they fell through quiescent air. This increases retention time in the tower and gives more time to solidify before reaching the bottom. At the same time, the air absorbs the heat from the falling prills.
In some cases, prilling is done into a liquid medium such as water as in the case of some sulfur prilling or in oil as in the case of some fertilizers. The water and/or oil provides the cooling medium. However, in the case of ammonium nitrate, which exemplifies one embodiment of this invention, and many other products such as urea, the prilling is done in air only. Ammonium nitrate and urea are both soluble in water, and ammonium nitrate and oil in combination are explosive.
The prills should be hard when they reach the bottom of the tower so that they will not deform or break. At the tower bottom, prills are collected by a number of methods such as: impinging on the bottom then removing with revolving scrapers; striking angled belting which funnels the prills onto a conveying means, usually a belt; and allowing the prills to fall into a fluid-bed which cools them further and conveys them out of the tower.
The air flowing past the falling prills removes heat and carries vapors from the prill surface. Since the air is extremely hot at the interface with the prill surface, but not away from the prill, small particles of submicron size are formed by recondensing and/or recombining vapors. Particles of this size are difficult to collect, and they present a highly visible and stable haze or fog which is very hard to remove from the exiting air. In the special case of ammonium nitrate, the fume is made up of a reaction of NH.sub.3 and HNO.sub.3 vapors which re-combine to form a blue haze consisting of ammonium nitrate particles of sub-micron size.
In the prior art, methods are known to avoid the emissions from prilling and the cost of subsequent clean-up operations of the air. Landis, in U.S. Pat. No. 4,190,622 used a combination of cocurrent and countercurrent flows to provide collection of emissions from a urea prill tower for subsequent removal from the air stream. Mahl et al. in U.S. Pat. No. 4,076,773, disclosed an internal recycle system whereby gases from the top of the tower were recycled to the bottom and ammonia was injected into the air to minimize the formation of ammonium nitrate fumes and allow clean-up. Others have disclosed various clean-up methods after the fumes and particulates have left the tower. The present invention reduces the need for accompanying clean-up devices and closed recycle towers and the expense of installing and operating them.
Also, heat transfer technology in Shirley et al., U.S. Pat. Nos. 4,213,924 and 4,424,176, both hereby incorporated by reference, in toto, describe water atomized in a rotary drum granulator to improve heat transfer from granules to air and improve granulation rates. In the latter, it was discovered that during cooling of solid particles such as in granulation, it was possible to get large increases in heat removal from the solid particles in the drum by controlled wetting of the particles just prior to forcing circulation air through a segment of the particles. The invention described herein takes that technology an order of magnitude further by actually atomizing water on a hot melt (still liquid) particle (prill) in such a manner that flash evaporation occurs before the water particles can be absorbed by the molten prill. The energy for evaporation of the water comes from the prill. For this to occur the melt must be sufficiently hot and the water particle sufficiently smaller than the melt particle such that the evaporation rate is extremely rapid, yet unlike that technology taught by others such as Haour et al. in U.S. Pat. 4,559,187 and Clark et al. in U.S. Pat. No. 4,339,401, the rapid cooling and water contact does not cause dissemination of the molten prill or particle. This process is also very different than a spray drag process such as described by Rauch et al. in U.S. Pat. No. 4,892,932 where water is sprayed in combination with the substance to be solidified or a prilling of water containing melt where water must migrate out of the granule, a process which cannot occur in a large prill in a short period of time such as is available in a prill tower.